118
8 Imaging
of the unsaturated and saturated star respectively, and I ref and I i are their integrated
counts; you can scale your exposure time such that in the revised exposure, the star is
unsaturated. Note that this is band specific and to some extent spectral type specific,
and uncertainties can be high:
I ref × 10
M ref −M i
2.5
= I i
(8.1)
8.5.2 Imaging Globular Clusters
Globular clusters are very different objects from open clusters. They are old, very
dense, found exclusively in the galactic halo, and in general, are considerably more
massive than open clusters. Unlike open clusters, no star formation has ever been
detected within a globular cluster, and the stars contained within have lower levels
of elements with atomic numbers greater than two (which in astronomy parlance are
known as metals, with the ratio between hydrogen and a metals being its metallicity),
indicating that they formed early in the evolution of the galaxy. These low metallicity
stars are known as population II stars, while the younger, higher metallicity, stars
such as those found in open clusters and also including stars such as the Sun, belong
to population I. There is a possibility that ancient, ultralow metallicity population
III stars exist, most likely as very low mass, and therefore slow-burning, M-Dwarfs,
but these have yet to be detected.
The challenges for imaging a globular cluster are very different from those of an
open cluster. As globular clusters are in the halo, they lie at much greater distances
than many open clusters, and any very bright star is therefore likely to be a foreground
star rather than a cluster member. However, the globular cluster is a very dense ball
of stars, perhaps up to several hundreds of thousands of stars with an average density
of perhaps one per cubic parsec. Although this does not sound significant, near
the centre of the cluster, the average distance between stars may decrease to a few
hundred astronomical units.
Consider the line of sight as we pass from the edge of the cluster towards the centre.
Near the edge, the number of stars within the line of sight is low, and individual stars
are seen. However, as we move towards the centre, the number of stars in the line of
sight increases to the point where we are no longer able to resolve individual stars.
The secret to imaging a globular cluster, therefore, is to enhance the detectable
stars near the edge whilst not saturating the core, where we are unable to resolve
individual stars. This we can achieve by taking multiple images and stacking them
in a nonlinear fashion. This can be used for general imaging and astrometry but not
for photometry, as it makes the pixel values nonlinear.
Figure 8.9 shows a science frame of the globular cluster M3 taken in clear (i.e.,
without a filter). The exposure time was 60 s, and no other action than the removal
of the bias, dark, and flat frames has been undertaken except for inverting the image.
We can clearly see individual stars near the edge of the cluster, but the centre appears
saturated.
8 Imaging
of the unsaturated and saturated star respectively, and I ref and I i are their integrated
counts; you can scale your exposure time such that in the revised exposure, the star is
unsaturated. Note that this is band specific and to some extent spectral type specific,
and uncertainties can be high:
I ref × 10
M ref −M i
2.5
= I i
(8.1)
8.5.2 Imaging Globular Clusters
Globular clusters are very different objects from open clusters. They are old, very
dense, found exclusively in the galactic halo, and in general, are considerably more
massive than open clusters. Unlike open clusters, no star formation has ever been
detected within a globular cluster, and the stars contained within have lower levels
of elements with atomic numbers greater than two (which in astronomy parlance are
known as metals, with the ratio between hydrogen and a metals being its metallicity),
indicating that they formed early in the evolution of the galaxy. These low metallicity
stars are known as population II stars, while the younger, higher metallicity, stars
such as those found in open clusters and also including stars such as the Sun, belong
to population I. There is a possibility that ancient, ultralow metallicity population
III stars exist, most likely as very low mass, and therefore slow-burning, M-Dwarfs,
but these have yet to be detected.
The challenges for imaging a globular cluster are very different from those of an
open cluster. As globular clusters are in the halo, they lie at much greater distances
than many open clusters, and any very bright star is therefore likely to be a foreground
star rather than a cluster member. However, the globular cluster is a very dense ball
of stars, perhaps up to several hundreds of thousands of stars with an average density
of perhaps one per cubic parsec. Although this does not sound significant, near
the centre of the cluster, the average distance between stars may decrease to a few
hundred astronomical units.
Consider the line of sight as we pass from the edge of the cluster towards the centre.
Near the edge, the number of stars within the line of sight is low, and individual stars
are seen. However, as we move towards the centre, the number of stars in the line of
sight increases to the point where we are no longer able to resolve individual stars.
The secret to imaging a globular cluster, therefore, is to enhance the detectable
stars near the edge whilst not saturating the core, where we are unable to resolve
individual stars. This we can achieve by taking multiple images and stacking them
in a nonlinear fashion. This can be used for general imaging and astrometry but not
for photometry, as it makes the pixel values nonlinear.
Figure 8.9 shows a science frame of the globular cluster M3 taken in clear (i.e.,
without a filter). The exposure time was 60 s, and no other action than the removal
of the bias, dark, and flat frames has been undertaken except for inverting the image.
We can clearly see individual stars near the edge of the cluster, but the centre appears
saturated.